Deteriorated asphaltic concrete roadways or the like have traditionally been repaired by overlaying new material or by removing some or all of the old pavement and replacing it with new paving material. This practice has become extremely costly. Petroleum based asphalt is expensive and the transporting of materials to and from the paving site adds substantial energy, equipment and labor costs.
Economies can be realized by recycling deteriorated old asphaltic concrete instead of hauling such material to a dump site and discarding it as has been a common practice in the past. Deteriorated asphaltic concrete is heated until the asphalt binder becomes liquid or semi-liquid. The heated material may then be remixed and relaid to form new pavement either at the original site or elsewhere. Little or no new paving materials may be needed and in many cases substantial savings in equipment and transportation costs can be effected.
My prior U.S. Pat. Nos. 4,319,856; 4,175,885; 4,252,459 and 4,252,487 describe methods and apparatus for recycling deteriorated asphaltic concrete in place at a roadbed or the like by using microwave energy preferably in combination with hot gas to heat the old pavement deeply and rapidly. The heated material is then remixed on the roadbed and recompacted to form a restored pavement.
Alternatively, old asphaltic concrete or at least an upper layer of such pavement can be broken up by cold milling, ripping or similar techniques. The fragmented material is then lifted from the roadbed and transported to a recycling plant where it is heated and remixed. The resulting hot-mix may then be returned to the original site for use in repaving operations or may be used to form new pavement at some other location.
Recycling operations of the kind in which fragmented asphaltic concrete is removed from the roadbed for reprocessing have heretofore been less efficient and more costly than is desirable and have been subject to other problems and limitations as well.
In hot-mix plants, aggregates or rock fragments of various sizes are typically heated by exposure to very high temperature combustion gases produced by burning a fuel. The aggregates may, for example, typically be given a prolonged exposure to gas temperatures of around 700.degree. F. (371.degree. C.) as heating efficiency and throughput are both enhanced by such high temperatures. Asphalt itself cannot be subjected to such temperatures without undesirable results. Asphalt breaks down when heated to high temperatures, producing hydrocarbon fumes at temperatures in the range from about 300.degree. F. (149.degree. C.) to about 500.degree. F. (260.degree. C.) and smoke at higher temperatures. Thus very high temperatures degrade asphalt and create pollution problems.
This difference in tolerance of high temperatures is easily accomodated where hot-mix is being produced from virgin aggregates and new asphalt. The asphalt is not added into the mix until after the aggregates have traveled through the highest temperature regions of the heating and mixing apparatus and have reached a point where gas temperatures are substantially lower. This is not possible where the raw material is fragmented old asphaltic concrete as in that case the asphalt is already present and bound together with the aggregates. Asphalt deterioration and pollution problems can be dealt with in the recycling of old asphaltic concrete by lowering processing temperature and increasing processing time but this reduces productivity and increases costs very substantially.
The problem is further complicated in that fragmented asphaltic concrete typically consists of loose pieces of varying size, ranging from less than a millimeter to several centimeters in diameter. A hot gas stream transfers heat into the surfaces of the concrete pieces. Heat is then conducted slowly towards the centers of the pieces. The larger the piece the longer it takes for internal temperature to approach the gas temperature. By the time the center of a one inch (2.54 cm) piece has reached a desirable hot-mix temperature such as 250.degree. F. (121.degree. C.) for example the centers of pieces smaller than about 3/8th inch (0.95 cm) have been at full gas temperature, such as 700.degree. F. (371.degree. C.) for example, and asphalt coatings are fuming, smoking and degrading.
One known practical solution to the above discussed problems is to combine the recycling of old asphaltic concrete with the manufacture of new hot-mix. Fresh uncoated aggregates are heated to above hot-mix temperatures by exposure to a very hot gas stream and thereafter, at a point where gas temperatures are substantially lower, both fresh asphalt and fragmented old asphaltic concrete are added in. The old asphaltic concrete pieces are heated by heat transfer from the new aggregates. Mixing of the old and new materials during the heat transfer period produces a high quality hot-mix.
Recycling by combining old and new materials in the manner described above resolves some problems at the cost of introducing others. The process is inherently only partial recycling. Substantial amounts of new aggregates and new asphalt are necessarily consumed although there may often be sufficient old asphaltic concrete available to meet the needs of current paving operations if it could be economically heated without the addition of new materials, without asphalt degradation and without generating difficult pollution problems.
Recycling by combining heated new aggregates with old asphaltic concrete chunks is particularly unsuited to the type of paving operation where old pavement is to be broken up, reprocessed in a movable heating and mixing device at or near the site and then relaid at the same site as repaving. Hauling in the necessary new materials adds substantially to costs and results in the production of more hot-mix than is needed.
Another known process for heating fragmented old asphaltic concrete in a drum mixer or the like is not subject to asphalt degradation, excessive pollutant generation nor any requirement for unneeded new materials but has been undesirably costly as heretofore practiced. In particular, microwave energy may be used to heat the material in a very rapid and uniform manner.
Microwave energy, which is not itself heat, penetrates into the material and converts to heat in a distributed manner throughout the volume of each rock fragment. Large chunks and small fragments of the concrete are heated at substantially the same rate. Because of differences in molecular structure, the microwave energy does not directly heat the asphalt content to a very appreciable extent but the asphalt is quickly heated and softened by heat transfer from the rock. Consequently, the temperature of the mass can be quickly and uniformly raised to the desired level without overheating of the asphalt component.
Economic considerations may have inhibited the use of microwave energy for the above described purposes. Microwave is an inherently costly heating medium as compared with generating heat by combustion of a fuel. Energy losses occur during the conversion of electrical power to microwave energy, in the transmitting of such energy to the substance which is to be heated and because of heat generation in the walls, access structures and other components of the heating apparatus itself instead of in the substance that is intended to be heated. Further energy losses occur in engines used to drive generators for supplying electrical power for installations where utility electrical power may not be available such as in travelable heating apparatus that may be used at non-urban roadways or similar paving sites of other kinds. Microwave equipment is costly itself in comparison with fuel combustion heating apparatus.
The technical advantages of microwave energy as a heating medium for recycling fragmented asphaltic concrete or the like could be more effectively realized by reducing the energy inefficiencies and high costs of the prior practice which are described above.
The present invention is directed to overcoming one or more of the problems discussed above.